Bacterial nutrition and Growth

introduction

Microorganisms use chemicals called nutrients for growth and development. They need these nutrients to build molecules and cellular structures. The most important nutrients are carbon, hydrogen, nitrogen, and oxygen. Microorganisms get their nutrients from sources in their environment. When these microorganisms obtain their nutrients by living on or in other organisms, they can cause disease in that organism by interfering with their host’s nutrition, metabolism, and, thus disrupting their host’s homeostasis, the steady state of an organism. Organisms can be classified in two groups depending on how they feed themselves.

1- Organisms that use carbon dioxide (CO2) as their source of carbon are called autotrophs. These organisms “feed themselves,” auto- meaning “self” and -troph meaning “nutrition.” Autotrophs make organic compounds from CO2 and do not feed on organic compounds from other organisms.

2- Organisms that obtain carbon from organic nutrients like proteins, carbohydrate, amino acids, and fatty acids are called heterotrophs. Heterotrophic organisms acquire or feed

on organic compounds from other organisms. Organisms can also be categorized according to whether they use chemicals or light as a source of energy. Organisms that acquire energy from redox reactions involving inorganic and organic chemicals are called chemotrophs.Organisms that use light as their energy source are called phototrophs A microorganism requires two chemical elements in order to grow. These elements are carbon and oxygen.

1-Carbon

Carbon is one of the most important requirements for microbial growth. Carbon is the backbone of living matter. Some organisms, such as photoautotrophs, get carbon from carbon dioxide (CO2).

2-Oxygen

Microorganisms that use oxygen produce more energy from nutrients than microorganisms that do not use oxygen. These organisms that require oxygen are called obligate aerobes. Oxygen is essential for obligate aerobes because it serves as a final electron acceptor in the electron transport chain, which produces most of the ATP in these organisms. An example of an obligate aerobe is micrococcus. Some organisms can use oxygen when it is present, but can continue to grow by using fermentation or anaerobic respiration when oxygen is not available. These organisms are called facultative anaerobes. An example of a facultative anaerobe is E. coli bacteria, which is found in the large intestine of vertebrates, such as humans. Some bacteria cannot use molecular oxygen and can even be harmed by it. Examples include Clostridium botulinum, the bacterium that causes botulism, and Clostridium tetani, the bacterium that causes tetanus. These organisms are called obligate anaerobes.

Anaerobic growth

Because anaerobic organisms can be killed when exposed to oxygen, they must be placed in a special medium called a reducing medium. Reducing media contain ingredients like sodium thioglycolate that attaches to dissolved oxygen and depletes the oxygen in the culture medium. In health clinics and hospitals, it is necessary to detect microorganisms that are associated with disease. Selective and differential media are therefore used. Selective media are made to encourage the growth of some bacteria while inhibiting others. An example of this is bismuth sulfite agar. Bismuth sulfite agar is used to isolate Salmonella typhi from fecal matter. Salmonella typhi is a gramnegative bacterium that causes salmonella. Differential media make it easy to distinguish colonies of desired organisms from nondesirable colonies growing on the same plate. Pure cultures of microorganisms have identifiable reactions with different media. An example is blood agar. Blood agar is a dark red/brown medium that contains red blood cells used to identify bacterial species that destroy red blood cells. An example of this type of bacterium is streptococcus pyogenes, the agent that causes strep throat. MacConkey agar is both selective and differential. MacConkey agar contains bile salts and crystal violet, which inhibit the growth of gram-positive bacteria,

Bacteria normally reproduce by a process called binary fission:

1. The cell elongates and chromosomal DNA is replicated.

2. The cell wall and cell membrane pinch inward and begin to divide.

3. The pinched parts of the cell wall meet, forming a cross wall completely around the divided DNA.

4. The cells separate into two individual cells.

Some bacteria reproduce by budding. A small outgrowth or bud emerges from the bacterium and enlarges until it reaches the size of the daughter cell. It then separates, forming two identical cells. Some bacteria, called filamentous bacteria (or actinomycetes), reproduce by producing chains of spores located at the tips of the filaments. The filaments fragment and these fragments initiate the growth of new cells.

Generation time

The generation time is the amount of time needed for a cell to divide. This varies among organisms and depends upon the environment they are in and the temperature of their environment. Some bacteria have a generation time of 24 hours, although the generation time of most bacteria is between 1 to 3 hours. Bacterial cells grow at an enormous rate. For example, with binary fission, bacteria can double every 20 minutes. In 30 generations of bacteria (10 hours), the number could reach one billion

Phases of growth

There are four basic phases of growth: the lag phase, the log phase, the stationary

phase, and the death phase.

A- Lag phase

In the lag phase there is little or no cell division. This phase can last from one hour to several days. Here the microbial population is involved in intense metabolic activity involving DNA and enzyme synthesis. This is like a factory “shutting down” for two weeks in the summer for renovations. New equipment is replacing old and employees are working, but no product is being turned out.

B- Log phase

In the log phase, cells begin to divide and enter a period of growth or logarithmic increase. This is the time when cells are the most active metabolically. This is the time when the product of the factory must be produced in an efficient matter. In this phase, however, microorganisms are very sensitive to adverse conditions of their environment.

C- Stationary phase

This phase is one of equilibrium. The growth rate slows, the number of dead microorganisms equals the number of new microorganisms, and the population stabilizes. The metabolic activities of individual cells that survive will slow down. The reasons why the growth of the organisms stops is possibly that the nutrients have been used up, waste products have accumulated, and drastic harmful changes in the pH of the organisms environment have occurred. There is a device called a chemostat that drains off old, used medium and adds fresh medium. In this way a population can be kept in the growth phase

indefinitely.

D- Death phase

Here the number of dead cells exceeds the number of new cells. This phase continues until the population is diminished or dies out entirely.

Nutritional types in bacterial metabolism
Nutritional type / Source of energy / Source of carbon / Examples
*Phototrophs / Sunlight / Organic compounds (photoheterotrophs) or carbon fixation (photoautotrophs) / Cyanobacteria, Green sulfur bacteria, Chloroflexi, or Purple bacteria
*Lithotrophs / Inorganic compounds / Organic compounds (lithoheterotrophs) or carbon fixation (lithoautotrophs) / Thermodesulfobacteria, Hydrogenophilaceae, or Nitrospirae
*Organotrophs / Organic compounds / Organic compounds (chemoheterotrophs) or carbon fixation (chemoautotrophs) / Bacillus, Clostridium or Enterobacteriaceae

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